1. Field of the Invention
The present invention relates to the field of photolithography to form integrated circuits and more particularly to the field of developing an irradiated photoresist.
2. Discussion of Related Art
Photolithography is used in the field of integrated circuit processing to form the patterns that will make up the features of an integrated circuit. A photoresist is employed as a sacrificial layer to transfer a pattern to the underlying substrate. This pattern may be used a template for etching or implanting the substrate. Patterns are typically created in the photoresist by exposing the photoresist to radiation through a mask. The radiation may be visible light, mid ultraviolet (G-line, I-line), deep ultraviolet (248 nm, 193 nm), extreme ultraviolet (EUV) light, or an electron beam. In the case of a “direct write” electron beam, a mask is not necessary because the features may be drawn directly into the photoresist. Most photolithography is done using either the “i-line” method (non-chemically amplified) or the chemical amplification (CA) method. In the i-line method, the photoresist becomes directly soluble when irradiated and may be removed by a developer. In the chemical amplification method the radiation applied to the photoresist causes the photo-acid generator (PAG) to generate a small amount of a photo-generated acid throughout the resist. The acid in turn causes a cascade of chemical reactions either instantly or in a post-exposure bake. In a positive tone photoresist the photo-generated acid will deprotect the compounds used to form the photoresist to make the photoresist soluble. If a PEB (Post-exposure bake) is not used the developer will serve to stop the acid from causing further reactions. In either situation, there is typically a time lag in between the initiation of the reactions by the photo-generated acid and the quenching of the acid by the developer. As illustrated in FIG. 1a, during this time lag the photo-generated acid in an irradiated region 110 of the photoresist 120 may diffuse into the regions 130 of the photoresist 120 that were not irradiated and cause a reaction in the regions 140. The width of the opening 150 formed by developing the photoresist 120 will be greater than desired due to the migration of the photo-generated acid during the lag time into the regions 140 of the non-irradiated portion 130 of the photoresist 120. The migration of the photo-generated acid into the non-irradiated portion 130 of the photoresist 120 may cause line roughness and loss of control of the critical dimensions of the features patterned by the photoresist. A chill plate may be used to minimize the migration of the photo-generated acid after a post-exposure bake. But, as the critical dimensions of the structures formed by photolithography become smaller, and particularly as the technology passes into the 45 nanometer node, a chill plate may no longer provide the control of the acid migration necessary to achieve the critical dimensions in this node.
The photoresist may be removed by a developer after the photoresist is deprotected by the photo-generated acid. The deprotection by the photo-generated acid increases the solubility of the resist so that it may be removed by a basic developer. FIG. 1b illustrates a basic developer 160 that has been applied to a photoresist 120 to develop the irradiated portion 110. An organic aqueous base such as tetramethylammonium hydroxide (TMAH) may be used as the developer 160 to remove the photoresist from the irradiated areas. But, as the technology moves to the 45 nanometer node, the dimensions of the structures patterned by a photoresist mask will become so narrow that the traditional aqueous base developer may not be able to access the narrow features with high aspect ratios of 2 or higher and may fail to fully develop the irradiated portions of the photoresist. FIG. 1b illustrates the incomplete development of a photoresist 120 by the developer 160 by the area 170 of the irradiated portion 110 that was not accessed by the developer 160. Additionally, even when the developer 160 is able to fully access the irradiated area 110 of the photoresist 120 the developer 160 may cause line collapse due to the high surface tension of the aqueous developer 160, also as illustrated in FIG. 1b. The aqueous base developers therefore also affect critical dimension control. Another drawback to using aqueous base developers is that copious amounts of the aqueous developer and water rinses to remove the aqueous developer are used, thus creating a large amount of waste.